The present invention generally relates to a system and method for improving the accuracy of an electromagnetic navigation system for use with medical applications. Particularly, the present invention relates to a system and method for improving the effectiveness of an electromagnetic shield assembly for use on a C-arm.
Electromagnetic type navigation systems are useful in numerous applications. One application of particular use is in medical applications, and more specifically, image guided surgery. Typical image guided surgical systems acquire a set of images of an operative region of a patient's body and track a surgical tool or instrument in relation to one or more sets of coordinates. At the present time, such systems have been developed or proposed for a number of surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac or other interventional radiological procedures and biopsies. Such procedures may also involve preoperative or intraoperative x-ray images being taken to correct the position or otherwise navigate a tool or instrument involved in the procedure in relation to anatomical features of interest. For example, such tracking may be useful for the placement of an elongated probe, radiation needle, fastener or other article in tissue or bone that is internal or is otherwise positioned so that it is difficult to view directly.
An electromagnetic tracking system may be used in conjunction with an x-ray system. For example, an electromagnetic tracking system may be used in conjunction with a C-arm fluoroscope. The C-arm fluoroscope may utilize an x-ray source at one end of the C-arm and an x-ray detector at the other end of the C-arm. The patient may be placed between the x-ray source and the x-ray detector. X-rays may pass from the x-ray source, through the patient, to the x-ray detector where an image is captured. The electromagnetic tracking system may generate an electromagnetic field between the ends of the C-arm and penetrate the body with minimal attenuation or change so tracking may continue during a surgical procedure.
One technique for generating the electromagnetic field involves using time-varying dipole fields. For example, dipole fields established by driving field-generating coils with an AC current signal. This approach allows synchronous demodulation of the induced signals and thus cumulate detected signal values to enhance sensitivity. Also, it allows the ability to establish the X, Y, and Z field components at different frequencies so that detected sensor output signals may be separated or demodulated simultaneously. This approach, however, has the disadvantage that varying magnetic fields induce eddy currents in the conductive structures found within the field. Induced currents themselves generate secondary magnetic fields, thus introducing distortions into the expected distribution. Conductive or ferromagnetic metal structures are generally commonly present in a medical tracking environment.
Once source of electromagnetic distortion in a C-arm environment is the x-ray detector. Historically, one technique to address the distortion from the x-ray detector is to mount a conducting structure about the x-ray detector. The conductor operates as a shield with respect to disturbances originating within the shield. The shield, which is typically structured as a metal can with openings at the top and bottom, then has a fixed position relative to one of the coil assemblies and may be effectively modeled. The eddy currents induced in the sheet metal cylinder by the magnetic field from the transmitter assembly, and the secondary field formed by these induced currents, may be modeled and accounted for in a distortion map.
Current shields, however, may allow signal leakage through seams, joints, and the x-ray detector window, for example. The signal leakage reduces the effectiveness of the shield. When the distortion map is created for calibration, it may take into account the signal leakage of the shield. In a situation in which a C-arm needs service or replacement parts, however, such parts may change the properties of the signal leakage (increase or decrease signal leakage, for example) through the shield. For example, if an image intensifier or flat panel detector is replaced, the properties of the signal leakage may be altered. If the properties of the signal leakage are altered, the distortion map may be mis-calibrated, resulting in improper operation of the tracking system.
The creation of the distortion map is typically performed using a robotics system during manufacturing and is a time consuming process. As the process for creating a distortion map is complicated and time consuming, it can be very costly in both monetary terms and in time, to perform on-site distortion mapping for calibration with new parts. Accordingly, a system and method is needed to minimize signal leakage from an electromagnetic shield. Such a system and method may minimize the need to recreate distortion maps, minimize equipment down time, and promote the interchangeability of replacement parts.